Gaseous emissions and grain-heavy metal contents in rice paddies: A three-year partial organic substitution experiment

To reduce the utilization of chemical fertilisers, which cause substantial nitrogen loss and widespread nonpoint source pollution, the application of organic manure has become an increasingly popular alternative in rice agriculture. It plays key roles in improving soil quality and maintaining rice yields, but its integrated impacts on trace gas emissions and heavy metal contents in rice grains remain poorly documented. We conducted a three-year field experiment with two application ratios (25% and 50%) of sewage sludge compost (S) and pig manure compost (P) during the rice season in eastern China. The emissions of methane (CH₄), nitrous oxide (N₂O), ammonia (NH₃), and the grain contents of nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb) were measured. Compared with urea, partial organic application, particularly 50%S and 50%P, led to a considerable increase in CH₄ emission (52%–71%), global warming potential (GWP, 50%–69%), and greenhouse gas intensity (46%–68%). However, it substantially decreased N₂O emission and NH₃ volatilisation, thus lowering the cumulative nitrogen loss by 32%–62%. Moreover, the average concentrations of Ni, Cu, Zn, Cd, and Pb in rice grains were 100–151 μg kg⁻¹, 2.31–2.78 mg kg⁻¹, 20.3–24.3 mg kg⁻¹, 44.3–123 μg kg⁻¹, and 8.69–15.2 μg kg⁻¹, respectively, which were significantly lower than food standard limits for rice in China. Both 25%S and 50%S achieved the highest grain yields while significantly decreasing grain Ni and Cd contents. Health risk assessment showed that the target hazard quotient of all the metals was <1 (0.006–0.73), and the hazard index that represents additive effects of pollutants was higher than the threshold, except for 25%S and 50%S. The results suggested 25%S as a potential fertilisation practice in rice fields that not only maintains low GWP and high yields but also seldom poses grain pollution or health risks.